Japanese Patent No. 2834465 discloses a power-supply system apparatus for vehicles. This power-supply system apparatus includes a pulse inverter, a bidirectional converter and a battery. The bidirectional converter is connected between the battery and the pulse inverter, steps up a voltage from the battery to supply the resultant voltage to the pulse inverter, and steps down a voltage from the pulse inverter to supply the resultant voltage to the battery.
The pulse inverter uses the voltage supplied from the bidirectional converter to drive an asynchronous machine. The asynchronous machine is used as an electric generator or starter.
Thus, when the asynchronous machine is used as an electric generator, the power-supply system apparatus converts an alternating-current (AC) voltage generated by the asynchronous machine into a direct-current (DC) voltage by means of the pulse inverter, and uses the bidirectional converter to step down the DC voltage as converted to supply the resultant voltage to the battery.
Further, when the asynchronous machine is used as a starter, the bidirectional converter steps up a DC voltage from the battery to supply the resultant voltage to the pulse inverter, and the pulse inverter converts the DC voltage from the bidirectional converter into an AC voltage to drive the asynchronous machine.
The conventional power-supply system apparatus, however, has a problem that, in the case where a voltage step-up operation for stepping up a DC voltage from the battery and a drive operation for driving the asynchronous machine as a starter are carried out at the same timing, excessive electric power is taken from the battery to the asynchronous machine.
There is also a problem that, in the case where a voltage step-down operation for stepping down a DC voltage from the pulse inverter and a drive operation for driving the asynchronous machine as an electric generator are carried out at the same timing, excessive electric power is brought from the asynchronous machine into the battery.
Accordingly, an object of the present invention is to provide a motor drive apparatus that can prevent input/output of excessive electric power to/from a power supply.